[llvm] [EarlyCSE] Rematerialize alignment assumption. (PR #109131)

[Nikita Popov via llvm-commits]

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@@ -1588,6 +1589,18 @@ bool EarlyCSE::processNode(DomTreeNode *Node) {
         if (InVal.IsLoad)
           if (auto *I = dyn_cast<Instruction>(Op))
             combineMetadataForCSE(I, &Inst, false);
+
+        // If the load has align metadata, preserve it via an alignment
+        // assumption. Note that this doesn't use salavageKnowledge, as we need
+        // to create the assumption for the value we replaced the load with.
+        if (auto *AlignMD = Inst.getMetadata(LLVMContext::MD_align)) {
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nikic wrote:

You also need `!noundef` for this transform. `!align` by itself only returns poison, while the assumption converts it into IUB.

https://github.com/llvm/llvm-project/pull/109131